The present invention relates to an antenna for radio apparatus, and more particularly to a compact antenna used for a compact portable terminal which occupies only a small volume, such as a portable mobile terminal. The invention also relates to an antenna of small size applied to a portable receiver or the like, and more particularly to an antenna of small size preferably applied to a portable receiver of small size because of its high mass productivity and its wide frequency band.
An inverted F type antenna or a helical antenna has hitherto been employed for compact portable terminals occupying a small volume, such as portable mobile terminals. In either type of antenna, when the antenna volume is decreased as the miniaturization of the terminal advances, impedance matching problems with the radio frequency section of the receiver become inevitable, and a capacitance component larger than a radiation resistance component takes place. In order to cancel this large capacitance component, the conventional antenna needs a matching circuit provided separately from the antenna proper. An example of such a conventional antenna is disclosed in, for example, JP-B-2-22563.
In the conventional antenna, characteristics of the antenna having the antenna proper and the matching circuit in combination have to be studied, and, from a standpoint of occupied volume, the matching circuit forms a factor which limits the miniaturization. Further, the matching circuit is realized with lumped constant elements (inductors and capacitances) or transmission lines, and upon incorporation of the antenna into the terminal, these elements must also be incorporated thereinto, thus considerably raising cost.
On the other hand, the frequency band can be widened by constructing such a matching circuit to be as simple as possible. In a known extreme case intended for widening the frequency band, the aforementioned matching circuit is not provided. Instead, a plurality of helical antennas having respectively different electric lengths are provided without making any electrical contact with each other. This causes multiple resonance owing to electromagnetic coupling of the helical antennas, so that the substantial frequency band of the helical antenna connected to a feeder can be widened.
An example of the latter case is described in detail in U.S. Pat. No. 4,772,895.